High frequency thickness shear mode acoustic wave sensor for gas and organic vapor detection

ABSTRACT

A thickness shear mode (TSM) sensor having a visco-elastic polymer coating and a fundamental frequency greater than 20 MHz useful for organic vapor or gas detection. The TSM quartz resonators at a fundamental frequency of 96 MHz were evaluated for their performance in organic vapor sensing applications and results were compared with the performance of 10 and 20 MHz resonators. These devices were produced by chemical milling of AT-cut quartz. Seven test organic vapors were utilized at concentrations ranging from 0.2 volume percent to 13.7 volume percent in the vapor phase. In all cases, the rubbery polymer polyisobutylene was used as a sensing layer. Detailed results for various sensor parameters such as sensitivity, baseline noise and drift, limit of detection, response and recovery times, dynamic range, and repeatability for the 96 MHz device were compared with those for 10 and 20 MHz devices. The test case of benzene/polyisobutylene was chosen to make these detailed comparisons. The 96 MHz device was found to be more sensitive than the lower frequency devices. Device sensitivity was dependent on the benzene concentration. Response and recovery times were smaller for the 96 MHz device. Response times decreased with analyte concentration. Sensor response was in reasonable agreement with the perturbation model of Sauerbrey at lower concentrations and deviated at the higher concentrations for the 96 MHz device. Higher frequency TSM devices can be very useful as organic vapor sensors both in detection and process monitoring applications. These devices have the advantages of simpler electronics, easier design and fabrication, well-developed models and good baseline stability when compared to other acoustic wave devices.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to currently pending U.S. ProvisionalPatent Application 60/703,371, entitled, “High Frequency Thickness ShearMode Sensors for Gas and Organic Vapor Detection”, filed Jul. 28, 2005,the contents of which are herein incorporated by reference.

FIELD OF INVENTION

This invention relates to quartz crystal microbalance systems. Morespecifically, this invention relates high frequency thickness shear modesensors for gas and organic vapor detection.

BACKGROUND OF THE INVENTION

Sensing elements in polymer-coated vapor sensors are often surfaceacoustic wave (SAW) devices. To overcome selectivity limitations, arraysof SAW sensors are used, employing a micro-sensor system format whichhas pre-concentration, chromatographic column resolution and arraydetection stages [1]. One reason for this choice of SAW devices is thehigher sensitivity afforded by these typically higher frequency devices.Thickness shear mode (TSM) devices, also called the quartz crystalmicrobalance (QCM), have been employed in many analytical applications,and as commercial thickness monitors in deposition equipment, however,their use in vapor and gas sensing is not common, although, someelectronic nose type systems do employ them in odor sensing, which isessentially a vapor sensing application [2-5]. Some advantages of theTSM devices include simpler electronics, better baseline stability, lessinvolved design and fabrication, and well-developed response models.

Most TSM applications so far have utilized devices in the 5-20 MHzfundamental resonance frequency range. Commercial resonators have beenmanufactured routinely for some time by thinning quartz plates bychemical milling to oscillate at fundamental frequencies of a littleover 100 MHz and stable oscillators at fundamental and overtonefrequencies are readily available. However, such higher frequencydevices have rarely been tested in sensing applications and never in gasphase applications [6]. A concern in utilizing TSM devices of greaterthan about 20 MHz fundamental frequency in sensor applications is thefragility resulting from the thin piezoelectric material that isnecessary for obtaining higher fundamental frequencies. To providemechanical stability, devices are fabricated by surrounding a thinoscillating region with a thick outer ring. In addition to thefragility, such thin plates (of the order of 2 micron for a 100 MHzquartz device) could lead to baseline stability and consequent limit ofdetection issues. Finally, the active sensor area is typically reducedby design limitations on the ratio of the oscillating region to theouter ring, and the absolute value of the oscillating region, leading tosensor performance issues that cannot be predicted easily.

SUMMARY OF INVENTION

Thickness shear mode (TSM) quartz resonators at a fundamental frequencyof 96 MHz were evaluated for their performance in organic vapor sensingapplications and results were compared with the performance of 10 and 20MHz resonators. These devices were produced by chemical milling ofAT-cut quartz. Seven test organic vapors were utilized at concentrationsranging from 0.2 volume percent to 13.7 volume percent in the vaporphase. In all cases, the rubbery polymer poly(isobutylene) was used as asensing layer. Detailed results for various sensor parameters such assensitivity, baseline noise and drift, limit of detection, response andrecovery times, dynamic range, and repeatability for the 96 MHz devicewere compared with those for 10 and 20 MHz devices. The test case ofbenzene/poly(isobutylene) was chosen to make these detailed comparisons,and the general conclusions were found to be similar with othersolvents. As expected, the 96 MHz device was found to be more sensitivethan the lower frequency devices. Device sensitivity was dependent onthe benzene concentration. An enhancement factor of 8 to 27 whencompared to the 10 MHz device was seen as the benzene concentrationranged from 0 to nearly 7 volume percent in the vapor phase.Significantly higher enhancements for the 96 MHz device were limited bydifficulty in coating thicker sensing layers without damping out theresponse. No significant improvement in the limit of detection was foundin going to higher frequencies due to increased baseline noise. Responseand recovery times were smaller for the 96 MHz device. Response timesdecreased with analyte concentration. Sensor response was in reasonableagreement with the perturbation model of Sauerbrey at lowerconcentrations and deviated at the higher concentrations for the 96 MHzdevice. Higher frequency TSM devices can be very useful as organic vaporsensors both in detection and process monitoring applications. Thesedevices have the advantages of simpler electronics, easier design andfabrication, well-developed models and good baseline stability whencompared to other acoustic wave devices.

According to one aspect of the present invention there is provided athickness shear mode (TSM) sensor. The TSM sensor includes a quartzcrystal having an oscillating region of reduced thickness surrounded bya outer region, wherein the outer region is comparatively thicker thanthe oscillating region, an electrode and a polymer sensing film incontact with the oscillating region of the quartz crystal. By employinga thicker outer region mechanical stability is imparted upon the sensorwhile allowing the desired frequency in the oscillating region. Incertain embodiments the polymer is a viscoelastic polymer. The polymercan polyvinyl acetate, polyvinyl pyrrolidone, polyisobutylene,polystyrene and polystyrenebutadiene. In an advantageous embodiment thepolymer is polyisobutylene. In certain embodiments the frequency of thepolymer coated resonator is greater than 20 MHz, 30 MHz, 40 MHz, 50 MHz,60 MHz, 70 MHz, 80 MHz or 90 MHz. In other embodiments the frequency ofthe polymer coated resonator is about 96 MHz. In yet other embodimentsthe frequency of the polymer coated resonator is between 20 and 200 MHz,50 and 150 MHz or 80 and 120 MHz. In still other embodiments thefrequency of the polymer coated resonator is about 96 MHz and thepolymer is polyisobutylene.

According to a second aspect of the present invention there is provideda method of gas or vapor sensing in an analyte. The method includes thesteps of (1) providing a thickness shear mode (TSM) sensor (2)contacting the TSM sensor with the analyte and (3) measuring thefrequency shift of the sensor upon contact with the analyte. Thefrequency shift is indicative of the presence or concentration in theanalyte of the gas or vapor to be sensed. The provided TSM sensorincludes a quartz crystal having an oscillating region of reducedthickness surrounded by a outer region, an electrode; and a polymersensing film in contact with the oscillating region of the quartzcrystal. The outer region of the quartz crystal is comparatively thickerthan the oscillating region. In certain embodiments the gas or vapor isan organic gas or vapor. The gas or vapor can be benzene, hexane,cyclohexane, heptane, dichloroethane, chloroform, and toluene. In anadvantageous embodiment the gas or vapor is benzene. In certainembodiments the polymer is a viscoelastic polymer. The polymer can bepolyvinyl acetate, polyvinyl pyrrolidone, polyisobutylene, polystyreneand polystyrenebutadiene. In an advantageous embodiment the polymer ispolyisobutylene. In certain embodiments the frequency of the polymercoated resonator is greater than 20 MHz, 30 MHz, 40 MHz, 50 MHz, 60 MHz,70 MHz, 80 MHz or 90 MHz. In other embodiments the frequency of thepolymer coated resonator is about 96 MHz. In yet other embodiments thefrequency of the polymer coated resonator is between 20 and 200 MHz, 50and 150 MHz or 80 and 120 MHz. In still other embodiments the frequencyof the polymer coated resonator is about 96 MHz and the polymer ispolyisobutylene.

According to a third aspect of the present invention there is provided amethod of fabricating a sensor to detect organic gas or vapor. Themethod includes the steps of the steps of (1) providing an AT-cut quartzcrystal, (2) milling the oscillating region of the crystal and coatingthe AT-cut crystal with a chemically-sorbent polymer. The coatedoscillating region of the crystal has a frequency greater than about 20MHz. In certain aspect of the invention the chemically-sorbent polymeris a viscoelastic polymer. The polymer can be polyvinyl acetate,polyvinyl pyrrolidone, polyisobutylene, polystyrene andpolystyrenebutadiene. In an advantageous embodiment the polymer ispolyisobutylene. In certain embodiments the frequency of the polymercoated resonator is greater than 20 MHz, 30 MHz, 40 MHz, 50 MHz, 60 MHz,70 MHz, 80 MHz or 90 MHz. In other embodiments the frequency of thepolymer coated resonator is about 96 MHz. In yet other embodiments thefrequency of the polymer coated resonator is between 20 and 200 MHz, 50and 150 MHz or 80 and 120 MHz. In still other embodiments the frequencyof the polymer coated resonator is about 96 MHz and the polymer ispolyisobutylene. In certain embodiments the AT-cut quartz crystalsinclude electrodes on opposing surfaces of the oscillating region of thecrystal. The electrodes can be circular electrodes fabricated in gold.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made tothe following detailed description, taken in connection with theaccompanying drawings, in which:

FIG. 1 is an illustration of a milled TSM device according to thepresent invention.

FIG. 2 is a schematic of a vapor generating apparatus.

FIG. 3 is a photograph of 10 (left), 20 (middle), and 96 (right) MHz TSMdevices.

FIG. 4 is a graph illustrating repeatable sensor responses for the (A)10, (B) 20 and (C) 96 MHz devices.

FIG. 5 is a graph illustrating a comparison of TSM sensor responses.

FIG. 6 is a graph illustrating the experimental mass sensitivities asfunctions of vapor concentration for the three devices (A) 10 and 20 MHzdevices (B) 96 MHz device.

FIG. 7 is a graph illustrating the (A) calibration and (B) sensitivityof the three sensors as functions of vapor phase concentration.

FIG. 8 is a graph illustrating the (A) response and (B) recovery timesfor the three sensors as functions of vapor phase concentration.

FIG. 9 is an illustration of a TSM sensor used to sense a vapor wherethe TSM sensor has a quartz layer (lower), an electrode (middle layer)and a polymer layer (upper layer in illustrated resonator).

FIG. 10 is a graph showing the 20 MHz response to benzene of a TSMsensor utilizing a PVA film.

FIG. 11 is a graph showing the 10 MHz response to benzene of a TSMsensor utilizing a PVP film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a higher frequency device with polymericsensing layers useful in organic vapor and gas sensing applications.Typical sensor parameters of sensitivity, dynamic range, response andrecovery times, baseline stability, frequency noise, limit of detection,and robustness were explored using an exemplary 96 MHz fundamental moderesonant frequency device. This device was compared to 10 and 20 MHzfrequency devices. Higher frequency devices such as these provide analternative technology in gas detection applications, as well as provideviable process concentration sensors for on-line monitoring. Devices ofthese different frequencies were of different sizes, to meet designparameters of mechanical strength, mounting, and other considerations.

1. INTRODUCTION

To construct gas or vapor sensors, chemically-sorbent films are commonlycoated onto TSM resonators. Chemical sensitivity and selectivity isimparted by attaching a thin film to the acoustically active region ofthe TSM device. Devices employed in this work were AT-cut quartzcrystals with circular gold electrodes on both sides. Because of thepiezoelectric properties of the quartz material, application of avoltage between the two electrodes at the surface results in a sheardeformation of the crystal. The quartz crystal then vibrates via thepiezoelectric effect and this vibrational motion results in thegeneration of a transverse acoustic wave that propagates across thethickness of the quartz crystal. The resonant frequency of the TSMdevice decreases with the crystal thickness when a standing wavecondition is met, as

$\begin{matrix}{t_{q} = \frac{\lambda_{q}}{2}} & (1) \\{v = \left( \frac{\mu_{q}}{\rho_{q}} \right)^{1/2}} & (2)\end{matrix}$

Here v is the velocity of sound in the quartz and t_(q) is the quartzthickness. Equation 2 gives the velocity, where ρ_(q) is the density(2.648 g cm⁻³) and μ_(q) is the shear modulus (2.947×10¹¹ g cm⁻¹ S⁻²) ofthe quartz. Since the quartz thickness is much larger than the electrodethickness, the electrodes are neglected when determining the resonantfrequency. Equations have been developed to yield expressions for thedependence of the resonant frequency on the mass changes occurringwithin films coated onto the TSM sensor. The Sauerbrey equation is onesuch relationship, valid for small mass loadings, such as thoseoccurring in vapor sensing applications:

$\begin{matrix}{{\Delta\; f} = \frac{2f_{r}^{2}\Delta\; m}{\left( {\mu_{q}\rho_{q}} \right)^{1/2}}} & (3)\end{matrix}$where Δf is the measured frequency shift, f_(r) is the resonantfrequency, and Δf is the film areal mass density (which can be relatedto the film mass) [7]. Changes in the film mass will cause frequencyshifts; these frequency shifts are dependent upon the film selectivityand the device sensitivity. In Sauerbrey's model, the sensitivity c_(f)is given by:

$\begin{matrix}{c_{f} = \frac{2\; f_{r}^{2}}{\rho_{q}v_{q}}} & (4)\end{matrix}$

Consequently, a commonly used 10 MHz AT-cut quartz crystal (with thepreviously mentioned physical properties) will have a mass sensitivityof 2.26×10⁸ Hz cm² g⁻¹. The addition of material with an areal densityof 4.42 ng cm⁻² will cause a 1 Hz shift in this resonator, which iseasily measurable using common electronics.

Although a 10 MHz TSM sensor already has a high degree of theoreticalsensitivity, it is obvious from equation (4) that this sensitivity canbe considerably increased by increasing the resonant frequency. For a 56MHz device, experimental sensitivity increases proportional to the 2.88exponent of the resonant frequency have been reported for liquid sensingwith the signal to noise ratio improving by a factor of 6.5[8]. This2.88 power dependence is greater than predicted by Sauerbrey's model,which is not applicable for liquid phase operation. Even withsensitivity increases predicted by the Sauerbrey model, which is morelikely valid for gas phase operation, a 100 MHz device will be 100 timesmore sensitive than a 10 MHz device, and twice as sensitive as a 100 MHzRayleighwave SAW device fabricated in quartz and responding via the massloading effect [9]. Achieving higher frequencies requires thinner quartzcrystals. This is difficult to accomplish using conventional fabricationtechniques. However, higher frequency devices can be produced byoperating at overtones of the fundamental resonant frequency or by usingmilled devices [10].

The plate thickness t_(q) determines the wavelength of the fundamental(n=1) and harmonic (n=3, 5, 7 . . . ) resonances as

$\begin{matrix}{f_{n} = \frac{nv}{2t_{q}}} & (5)\end{matrix}$where f_(n) is the frequency of the n^(th) harmonic. Milled crystals aremade thinner only in the center so that a thin quartz membrane isfabricated with a thick outer ring allowing for mechanical stability.The design of high frequency resonators by milling techniques has beenstudied extensively [11-14]. These higher frequency devices are longknown to have improved mechanical stability and frequency-to-noiseratios at are better than other acoustic wave devices [15]. Chemicallymilled devices specifically designed for the experiments belowexperiments, with fundamental frequencies of approximately 96 MHz havingapproximately 20 mils electrode diameter were fabricated at MTronPTI,Orlando, Fla. A diagram of a milled TSM device is shown in FIG. 1.

Example High Frequency TSM Sensor

2.1 Apparatus Set-up

Organic vapor samples were generated at specific concentrations to testthe coated TSM sensors. Among the available techniques for generatingstandard gases, a dynamic method of generating vapor samples was used[16-18]. The equipment is fully automated and a schematic diagram isshown in FIG. 2.

Organic liquids were contained in four bubbler units housed in atemperature bath. MKS mass flow controllers were used to regulatenitrogen carrier gas through the bubbler units. Each bubbler unitconsisted of a flask in which the organic liquid was contained. Multiplemass flow controllers (100 sccm, and 200 sccm) allowed for variation oftest sample concentrations. Three streams were used: The carrier streampassing through the bubbler, and the diluting streams. The vaporpressure of the liquid at the bubbler temperature was calculated usingan accurate vapor pressure correlation (Wagner's equation) [19]. Themole fraction was approximated from the volume of the vapor generated,assuming saturation of the exiting vapor from the bubblers. Thegenerated vapor was further diluted and a new diluted mole fraction wascalculated. This mole fraction was equated to the volume fraction andthen the concentration was approximated using the ideal gas law.Activities of the solvents in PIB, calculated using these experimentaldata, matched very well with literature values, indicating that thevapor concentrations were being generated accurately [20]. Furthercalibration of the dilution system using charcoal traps and weighingalso indicated that the vapor generation system functioned as designed.Generated vapor concentrations were estimated to be well within 10% oftrue values. An Agilent 4294A precision impedance analyzer was used tomonitor the resonant frequency and equivalent circuit parameters of theTSM devices. A custom-made stainless steel test cell was designed forhousing the sensor and was kept under temperature control. The TSMdevice was attached to a printed circuit board using a commerciallyavailable socket holder, obtained from International CrystalManufacturing, Oklahoma City, Okla., for 10 MHz and 20 MHz resonatorsand a transistor socket for the higher fundamental frequency resonators(96 MHz). The sockets facilitated easy removal of the sensors withoutdisturbing electrical connections.

2.2 Coating and Chemicals

TSM devices were coated by spray coating utilizing an air brush [21].Poly(isobutylene) (Acros) was used as the sensing film. The polymer hadan average molecular weight of 400,000. A dilute solution of the polymer(0.1%), dissolved in chloroform (99.9% HPLC), was aspirated through anatomizing nozzle using compressed nitrogen gas. As the atomized dropletsimpact the device surface, the volatile solvent is evaporated with aheat gun to leave the polymer coating. Polymer coatings formed usingthis method may have irregular texture and coverage. However,thicknesses were reproducible because the device frequency was monitoredthroughout the coating process. The resonators were soaked in chloroformand cleaned in a Harrick plasma cleaner prior to coating. The polymerwas coated to equal thickness on each side of the TSM resonators toachieve various frequency shifts corresponding to different filmthicknesses. The devices were allowed to air dry after each coatingapplication, and were cured to anneal the film. Unless handled withcare, the 96 MHz frequency resonators were found to be fragile andshattered during spray coating. Upon refining the coating process,devices were handled without damage through several cycles of coating,experimentation and cleaning. The frequency and equivalent circuitparameters of the uncoated device were recorded before and afterapplication of the polymer. These parameters of the device were alsomonitored with time throughout sorption and desorption of the vapors inthe polymer.

A TSM device is only a resonator if there is no sensing film. Thesensing film imparts selectivity and sensitivity towards a particularchemical, thus, making a resonator into a sensor. The sensing films usedwere polymer coatings. The coating should physically or chemically bondto the surface of the TSM resonator and should behave as an ideal masslayer. An ideal mass layer should be infinitesimally thick when comparedto the thickness of the quartz material (ignoring the thickness of thegold electrodes). The polymer coatings were applied to both sides of theelectrodes, but the total film thickness was calculated as the sum ofthickness on both sides of the resonator. In effect the actual filmthickness is the half of the total film thickness. Since differentresonators were compared in this study, the physical characteristics ofthe resonators were different, with each sensor having a differentelectrode area. Since only the coated electrode area is region of thesensor surface where the coating analyte interaction (physisorbtion)occurs, comparing the sensitivity of devices with different electrodeareas is inaccurate. The model developed equation (11) accounts forthese differences, however, the polymer film coating Δf_(p), must besimilar for all devices. Since each device has a different sensitivity,the frequency shift due to the polymer coating will be different, butthe thickness can be constant. Another complication to having similarfilm thickness, however, is introduced because each device has adifferent quartz blank thickness. Consequently, the ratio of thethickness of the quartz blank to the polymer film must be kept constant.This ratio was approximately 0.5%. The uniformity of the film has littleeffect on the detection of the chemical vapors, however the film shouldbe adherent and stable in the presence of the organic vapors. Severalmethods are available for coating TSM resonators including spin coating,spray coating, drop coating and coating using an oscillating capillarynebulizer. The performance of several polymer film coatings ofpoly-vinyl acetate, poly-vinyl pyrrolidone, polystyrene-butadiene, andpoly-isobutylene were. The results based on several performancecriterion were used to select the appropriate film. Since, we weresensing for organic vapors, rubbery polymer films known to physisorbwith organic vapor, such as the ones mentioned previously, wereselected. Nevertheless, some films perform better for a givenapplication and polymer properties, such as glass transitiontemperatures, which should be taken into account when choosing thesensing film.

2.3 The TSM Devices

All crystals were AT-cut quartz with gold electrodes on a chromiumadhesion layer. Some parameters of the devices are given in Table 1. Themilled membrane diameter of the 96 MHz device was approximately 0.127cm. The 10 and 20 MHz devices were obtained from International CrystalManufacturing. The 96 MHz devices were specially fabricated at MTronPTI,Orlando, Fla., using chemical milling. The resonator quartz blanks wereetched in NaOH.0.5H₂O at 180° C. Additionally, these resonators werefabricated with ring thicknesses of approximately 50 μm and membranethicknesses of approximately 17 μm. FIG. 3 is a photograph of themounted, chemically milled 96 MHz resonator, along with the 10 and 20MHz devices used for comparison.

TABLE 1 Parameters of the TSM devices Blank Electrode Electrode MilledResonator Diameter Diameter Thickness Electrode Area (MHz) (cm) (cm) (Å)Material (cm²) 9.98 1.376 0.5105 1000/100 Au/Cr — 19.97 0.8077 0.34801000 Au/Cr — 96.89 0.508 0.0508 — Au/Cr 0.0507

3. PERFORMANCE TESTING OF FABRICATED DEVICES

Performance of the TSM device with a poly(isobutylene) sensing filmtested with benzene analyte is presented below. This typical case of anorganic solvent and a rubbery polymer is chosen to evaluate thedependence of sensor parameters on frequency. Each of the resonators inTable 1 was exposed to various concentrations of benzene. The frequencyresponses of the sensors were recorded during exposure to compare thesensitivities, frequency noise, limits of detection, response andrecovery times, and dynamic range for the three devices.

The vapor phase concentration was increased from 27,557 mg/m³ to 232,688mg/m³, with purges of pure nitrogen gas between each exposure to allowthe benzene vapor to desorb and the film to recover. A total flow rateof 100±1 sccm was always maintained over the surface of the device. Thetemperature of the cell was maintained at 22.5° C. and the benzene vaporwas generated at 15° C. Temperature fluctuations of the cell were within±0.1° C. Frequency measurements taken using Labview and the Agilentimpedance analyzer were stable to within ±1 Hz.

Film thickness of the coated polymer and associated frequency shift foreach resonator is given in Table 2. The ratio of the polymer film to thequartz membrane thickness is kept below 1% to stay within the massbalance regime within which Sauerbrey's equation is known to be validfor inertially coupled layers. Also, sensor response dampens as thepolymer film thickness is increased, such that a limit of sensing filmthickness exists for each sensor of a given fundamental resonantfrequency. Since different resonators have different blank thicknesses,the thickness of the polymer sensing film cannot be constant. However,the ratio of the thickness of the quartz blank to the thickness of thefilm can be kept at a near constant ratio to facilitate more rationalcomparison of sensor response parameters. Due to the higher sensitivityof the 96 MHz device, this constancy was difficult to achieve, leadingto a slightly higher value, but still within 1%. In the experimentsconducted, the responses of each resonator to various concentrations ofbenzene were recorded. From these data various sensor responseparameters were determined and compared.

TABLE 2 Polymer film parameters Resonator Film Frequency ThicknessFrequency Thickness Shift Ratio (MHz) (nm) (kHz) (percent) 9.993250 96120,112 0.58 19.993400 420 34,440 0.50 96.999522 141 275,413 0.82

3.1 Sensor Response, Repeatability, and Dynamic Range

The resonator responses (frequency changes) due to exposure to theanalyte are repeatable as shown in FIG. 4. Three trials were conductedwith each device. Each run consisted of 1200 seconds of an initial purgewith ultra high purity nitrogen, after which exposure and purge timeswere of 600 seconds duration each. The procedure was chosen todemonstrate the viability of the resonator as a sensor in terms ofrecovery after exposure to the test sample. Repeated cycling andre-testing of the polymer coated devices after several weeks yielded thesame results. The inherent increase in sensitivity of the 96 MHzresonator, due to the increase in resonant frequency, is demonstrated bya comparison of the sensor response from each resonator, as shown inFIG. 5.

A noticeable drift in the baseline frequency was observed for allresonators. The base line drift for the 10 MHz device over the durationof the entire experiment of 12,000 seconds was 49 Hz. The 20 MHz and 96MHz resonators had higher baseline drifts of 260 Hz and 2342 Hz,respectively. The baseline drift was probably the result of dewettingeffects. De-wetting affects the shape of a thin film polymer by reducingthe area of the film/surface interface. During exposure to the analytethe polymer film likely breaks up into beads of isolated droplets. Thiseffect was also observed during the coating procedure and has also beennoted in previous studies [22]. The frequency of the polymer coatedresonators was observed to increase slightly when left overnight. Thiswas probably due to absorption of water vapor from the air.Consequently, the initial rise in resonant frequency during firstexposure to pure nitrogen is due to de-sorption of the water.

Table 3 shows the dynamic range of seven organic vapors (benzene,hexane, cyclohexane, heptane, dichloroethane, chloroform, and toluene)which were tested with these poly(isobutylene) coated resonators. Theseranges are useful in considering the TSM devices as process streamconcentration monitors. All these organic solvents are reasonablysoluble in the rubbery polymer poly(isobutylene), although frequencyshifts at exposures of same concentrations are different for differentorganics, allowing for possibility of discrimination. However, the focusof this study is to evaluate the higher frequency 96 MHz device forpossible improvements over the lower frequency devices in a comparativestudy of the three devices. Benzene analyte sensor response parametersare presented as a typical case. Sensor response parameters for theother organics from Table 3 led to similar conclusions.

TABLE 3 Dynamic range of TSM devices for several organic vapors DynamicRange (Volume percent) Chemical 10 MHz 20 MHz 96 MHz Benzene  0.8-7.0 0.8-7.7  0.8-7.0 Toluene  0.2-1.3  0.2-1.5  0.2-1.5 Hexane  1.4-9.2 1.4-11.5  1.4-9.2 Heptane  0.4-2.5  0.4-3.2  0.4-2.9 Cyclohexane 0.9-6.5  0.9-8.0  0.9-6.5 Dichloroethane  0.7-5.8  0.7-6.4  0.7-6.4Chloroform  2.0-13.7  2.0-15.2  2.0-13.7

3.2 Sensitivity

Sauerbrey's model (equation 3) can be utilized to calculate thetheoretically expected sensitivity for this polymer sorption process foreach of the tested devices, provided the conditions for its validity aremet, viz, the polymer/solvent adlayer should behave as a rigid, inertialmass perturbation to the quartz thickness. This model provides anexpression for frequency sensitivity to areal mass density (mass/unitactive area of crystal), however, the active area is not known exactly.Hence, comparison with theory becomes convenient if an experimentalsensitivity to areal mass density were expressed in terms of easilymeasured parameters. To establish if each of the three devices testedbehaves according to Sauerbrey's model, we derive an expression for theexperimental sensitivity in the following way:

The frequency shift due to the polymer and sorbed solvent in the sensingfilm for each exposure is:Δf=Δf _(p) +Δf _(s)  (6)where Δf_(p) is the frequency shift due to the polymer and Δf_(s) is thefrequency shift due to the solvent (analyte). Similarly, the areal massdensity (mass/unit area) of the polymer/solvent sensing layer is givenby:Δm=Δm _(p) +Δm _(s)  (7)

We can define weight fraction of solvent in the polymer, w, in terms ofthe areal mass densities as:

$\begin{matrix}{w_{o} = \frac{\Delta\; m_{s}}{{\Delta\; m_{p}} + {\Delta\; m_{o}}}} & (8)\end{matrix}$

Rearranging for Δm_(s)+Δm_(p)

$\begin{matrix}{{{\Delta\; m_{s}} + {\Delta\; m_{p}}} = {\Delta\; m_{p}\;\left( \frac{1}{1 - w_{s}} \right)}} & (9)\end{matrix}$

Noting that the polymer film areal mass density equals hp, the productof the film thickness and polymer density, we have

$\begin{matrix}{{{\Delta\; m_{p}} + {\Delta\; m_{s}}} = {h\;\rho_{p}\;\left( \frac{1}{1 - w_{s}} \right)}} & (10)\end{matrix}$

The sensitivity becomes

$\begin{matrix}{c_{p} = \frac{{\Delta\; f_{p}} + {\Delta\; f_{s}}}{h\;\rho_{p}\;\left( \frac{1}{1 - w_{s}} \right)}} & (11)\end{matrix}$

This expression can be utilized for calculating experimental masssensitivity as it involves no determination of actual mass loaded on thecrystal, which is difficult to determine given that the active areas ofthe devices at different frequencies are both difficult to establish andare numerically different (due to different fabricated sizes). Since themass involved is very small, there is no other easy technique availableto weigh this loaded mass, necessary for establishing experimentalsensitivity. However, equation 11 involves solvent weight fraction,which can be easily established from measurements taken with the 10 MHzdevice, provided this device behaves as a mass balance. This same weightfraction is applicable to the 20 and 96 MHz devices, for exposures atsame vapor phase concentrations, if we assume that thermodynamicequilibrium is achieved between the polymer film and the vapor phase. Inthe experiments, the mass balance regime of the 10 MHz device wasestablished by measurements of the equivalent circuit parameters, wherethe resistance values were reasonably constant for exposures at variousconcentrations tested. Thermodynamic equilibrium was confirmed bycomparison of the measured solvent weight fractions to those from otherliterature thermodynamic measurements which utilized totally differentsorption techniques [20]. The weight fractions were determined fromfrequency shifts by application of Sauerbrey's model to this 10 MHzdevice via

$\begin{matrix}{w_{s} = {\frac{\Delta\; m_{s}}{{\Delta\; m_{p}} + {\Delta\; m_{s}}} = \frac{\Delta\; f_{s}}{{\Delta\; f_{p}} + {\Delta\; f_{s}}}}} & (12)\end{matrix}$

Finally, the polymer thickness h was determined by profilometermeasurements, as well as from measured frequency shifts Δf_(p) andapplication of Sauerbrey's model for each device, before exposures tosolvents. Poly(isobutylene) density was taken as 0.92 gm/cm³, as givenby the supplier Acros.

Experimental sensitivities for the devices at the three frequencies areshown in FIG. 6. These sensitivities are seen to be concentrationdependent, more so for the 96 MHz device. The much larger sensitivity ofthis device in comparison to the lower frequency devices is shown inFIG. 6. Sauerbrey's model predicts 4-fold and 92-fold increases in arealsensitivity for the 20 and 96 MHz devices, respectively, in comparisonto the 10 MHz device. The ratios range from 4.01 to 4.05, and 111 to126, for the 20 and 96 MHz devices, respectively (FIG. 6). From linearregression at each vapor exposure concentration, we can establishwhether the f² dependence predicted by Sauerbrey's model is borne out.We find that the exponent is close to 2, varying between 2.08 to 2.14,in going from the lowest to the highest exposure concentrations ofbenzene vapor. Note that the highest exposure concentration representsabout 7 volume % benzene in the vapor phase, which corresponds to 17weight % benzene in the polymer film. At the lower concentrations, theSauerbrey model is followed well for all three devices. However, at thehigher concentrations, with increased weight fractions of benzene in thepolymer, the higher frequency device deviates to some extent from themodel.

FIG. 7 shows the calibration and device sensitivity plots as functionsof vapor phase concentration for the three sensors studied. The devicesensitivity is the first derivative of the calibration curve, and issimply the change in device frequency per unit change in vapor phaseconcentration. The calibration curves are represented very well by aquadratic function of the formΔf=aC ² +bC  (13)

Where Δf is the frequency shift that results from solvent exposure, andC is the vapor phase concentration of the analyte. Parameters for theseregressions are given in Table 4.

TABLE 4 Sensor calibration equation parameters Regression Sensor DeviceA B Coefficient 10 MHz 5.390 × 10⁻⁸ 5.439 × 10⁻³ 0.9974 20 MHz 9.839 ×10⁻⁸ 1.008 × 10⁻³ 0.9976 96 MHz 1.661 × 10⁻⁶ 4.280 × 10⁻² 0.9921

Device sensitivity is seen to be linear in concentration, with a largerslope for the 96 MHz device indicating that this device can be utilizedvery well as a process monitor for higher concentrations of analyte, inaddition to being useful as a low concentration detector (such as inlower explosion limit detection safety applications). Note that thedevice sensitivity of the 96 MHz sensor is 7.9 times that of the 10 MHzdevice at the lowest end of the concentration range, and is nearly 27times that of the 10 MHz device at the highest concentration studied(2.33×10⁵ mg/m³). The device sensitivity ratio can be estimated fromSauerbrey's model to be a factor of 92 times the ratio of the filmthicknesses, which are 961 and 141 nm, for the 10 and 96 MHz devices,respectively. This yields a constant value of 13.5 across theconcentration range. In contrast, the observed values vary from 7.9 to27. Part of the discrepancy at the lower concentration end can beexplained by the uncertainty in determining the polymer film thickness,especially on the 96 MHz device. At the higher end, it is the deviationof the device response characteristics from Sauerbrey's model(viscoelastic effects) for the 96 MHz device that likely explain thedifference. Given the much thinner quartz thickness of the 96 MHzdevice, it is difficult to achieve much larger areal mass densities ofthe polymer (coating thicknesses) without damping out the response. Wehave found that the poly(isobutylene) film thickness can at most beincreased by an additional 30% without damping out the device. Hence, itshould be accepted that the improvements to device sensitivities for thehigher frequency TSM devices will be factors of 10 to 40 compared to thelower frequency devices for organic vapor sensing applications. This isindeed a significant improvement, with further optimization at anintermediate frequency possible. Larger thicknesses of polymer coatingscould also come with the penalty of increased response and recoverytimes, due to slow diffusion of solvents in polymers.

3.3 Limit of Detection

The limit of detection (LO.D.) and the noise level for each resonatorwere also determined from the sensor responses. Similar to the baselinedrift, the frequency noise increased with the fundamental frequency ofthe resonators. The frequency noise is important since it determines thedetection limit for the sensor. The frequency noise was defined as threetimes the standard deviation (S.D.) of the mean resonant frequency takenover a 7 minute interval in the presence of 100 sccm of pure nitrogengas flow. L.O.D., signal noise, and baseline drift for each resonatorare given in Table 5. The limiting slopes at zero vapor concentration(intercepts of the sensitivity lines in FIG. 7) were utilized indetermining L.O.D.

TABLE 5 Comparison of sensor drift, noise and LOD Total Noise L.O.D.Resonator baseline drift (Hz, 3S.D. (mg/m³, (MHz) (Hz/220 min) meansignal) 3Noise/Cp) 9.98 49 0.444 245 19.97 260 0.900 268 96.89 23423.650 256

The noise level for the 96 MHz resonator was considerably higher thanthat of the 10 MHz resonator. Consequently, the L.O.D. did not improvedespite the improvement in the sensitivity of higher fundamentalfrequency resonators. It is typical and expected that higher sensitivityphysical structures are prone to higher baseline noise leading to lowerL.O.D.s. It is likely that the inert nitrogen gas flow is the cause forthe environmental perturbations that lead to the baseline noise. Indeed,noise levels were found to be considerably lower for this higherfrequency device in a static environment. In a lower explosion limit orother detection application, air flow over the device is minimal andlower L.O.D.s can be realized in practical devices. Another source ofnoise is likely from the electrical interference on these devices, whichwill be further minimized in a circuit board mounted device driven usingan oscillator circuit.

Considering that the noise levels in a flowing stream are similar inmagnitude at all exposure concentrations, and putting this together withnearly 30-fold increase in sensitivity at higher concentrations, we canconclude that the higher fundamental frequency devices will make goodprocess monitors. High sensitivity leading to high accuracy is desirablefor process control applications. By dilutions using an inert gassupply, the dynamic range can be expanded to arbitrarily higherconcentrations in the analyzed gas stream.

3.4 Response and Recovery Times

Sensor frequency shift did not follow an exponential decay with time,hence, response time constants using fits to an exponential functioncould not be made. Instead, a value for the full response wasestablished from a criterion of fluctuations around a mean value, andtime constants for 63%, 95% and 99% sensor response were calculated. The63% and 99% response times as well as the 99% recovery times are shownin FIG. 8. The 63% response time is between 10 and 20 seconds for allthree sensors, except at the lowest concentration tested, where, thehigher frequency device responds more quickly. The 99% response timesalso show a decreasing trend with concentration for all devices, withthe 96 MHz device showing smaller times. These results are consistentwith unsteady-state diffusion behavior in a polymer slab, however,smooth and monotonic decrease of the response times with increasingbenzene concentration is not seen. This could be due to the non-uniformnature of the polymer film, and the complex nature of the diffusionprocess in these films. Recovery is relatively quick (FIG. 8.b), with100 to 180 seconds for 99% recovery seen for all devices over theconcentration range studied, with the 96 MHz device recovering quickerthan the lower frequency devices.

3.5 Alternative Polymer Films

Several polymer films were utilized for sensing organic vapors. Thesepolymers were polyvinyl acetate, polyvinyl pyrrolidone, polyisobutylene,polystyrene, polystyrenebutadiene copolymers. An ideal sensing filmshould be able to recover after exposure to the analyte, have a stablebaseline frequency, and repeatable responses. These features areaffected by the properties of the polymer. Mainly the film should berigid enough to move with the oscillation of the TSM resonator, but thefilm should also be soft enough to allow for sorption of the analyte.Films that are too rigid have longer equilibrium times, consequently,these films may not be practical for a sensing application. Equilibriumtimes of less than one minute are ideal. Polystyrene, polyvinyl acetate,and polyvinyl pyrrolidone films were found to have equilibrium times ofmore than 20 minutes when exposed to 27,000 mg/rn3 of benzene at roomtemperature. FIG. 11 shows a typical response of a 20 MHz TSM resonatorcoated with 23.25 kHz of polyvinyl acetate (PVA) to benzene vapors(0.823 to 7.66 volume percentages). Notice that the resonator frequencycontinuously decreases and that an equilibrium is never reached for allexposure levels. This response is typical of a film that has a longequilibrium time and does not readily desorb the analyte. A similarresponse was obtained with polyvinyl pyrrolidone (PVP) coated to 20.03kHz on a 10 MHz resonator (FIG. 11). Hence, polystyrene would not be anideal film for sensing benzene. Polymer films of polybutadiene were toosoft, resulting in unstable baseline frequencies, large baseline drifts,and poor repeatability.

The polymer glass transition temperature was found to be factor whichdetermined whether a film would behave ideally. Glass transitiontemperatures of the polymers investigated in this work are presented inTable 6. The temperature at which sensing occurs should be above theglass transition temperature of the polymer for equilibrium times to below. This is because the as the temperature increases the thermal energyin the polymer solvent system is sufficient to overcome molecular forcesbetween the polymer, allowing for sorption. However, too low of a glasstransition temperature resulted in a polymer film that was difficult tocoat onto the TSM resonators. Additionally, as the polymer sorbs theanalyte, properties of the polymer change. In particular the glasstransition temperature becomes depressed, depending upon theconcentration of the analyte. This is because the analyte has alubricating effect on the polymer and causes the individual chains inthe polymers to move more freely. The net effect is a plasticization ofthe polymer and a depression of the glass transition temperature.Changes in the shear modulus of the polymer also result from sorption.This is because the viscoelastic properties of the polymer changes inresponse to the sorbed vapor. The shear modulus is directly related tothe rigidness of the polymer and the intermolecular forces within thepolymer. Consequently, as these forces change due to sorption, the shearmodulus also changes. The motional resistance of the TSM resonatorincreases as the polymer film becomes softened. This change in themotional resistance can be used to determine the shear modulus. Sincethe extent of change in the viscoleastic properties of the film variesaccording to the quantity and identity of the organic vapor, it waspossible to distinguish between the organic vapors by monitoring themotional resistance of the TSM resonator.

TABLE 6 Polymer T_(g) ( ° C.) Polybutadine −102 Polyisobutylene −76Polystyrene 60-93 Polyvinylacetate 30 Polyvinylpyrrolidone 160

4. CONCLUSIONS

Milled TSM resonators with increased fundamental resonant frequenciescan be utilized as improved organic vapor sensors both in a detectionmode and in process stream monitoring applications. Significantimprovements to the device sensitivity are realized compared to thelower frequency devices. Sensitivity was found to be analyteconcentration dependent. Further improvements to sensitivity were foundnot to be possible due to damping out of the response from the higherfrequency devices at higher sensing film thicknesses. L.O.D. for thehigher frequency device was found to be comparable to the lowerfrequency devices. Response times were shorter for the higher frequencydevice, with a decreasing trend with analyte concentration for alldevices. Recovery times were small for all devices and increased withanalyte concentration. These TSM devices coated with rubbery polymerswere found to exhibit adequately fast response and recovery times.Sensor frequency response magnitudes compared reasonably well with theperturbation model of Sauerbrey, with larger deviations observed athigher vapor concentrations. Fragility of the milled device was foundnot to be a significant issue with properly designed polymer coatingprocedures. With further optimization of device frequency and polymerfilm thickness, viable organic vapor sensors are possible for detectionand process monitoring applications using higher frequency TSM devices.These devices have advantages of simpler electronics, easier design andfabrication, well-developed models and good baseline stability comparedto other acoustic wave devices.

REFERENCES

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The disclosure of all publications cited above are expresslyincorporated herein by reference, each in its entirety, to the sameextent as if each were incorporated by reference individually.

It will be seen that the advantages set forth above, and those madeapparent from the foregoing description, are efficiently attained andsince certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatters contained in the foregoing description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween. Now that theinvention has been described,

1. A thickness shear mode (TSM) gas and organic vapor sensor comprising:a quartz crystal having an oscillating region of reduced thicknesssurrounded by an outer region, wherein the outer region is comparativelythicker than the oscillating region and the oscillating region has anoperating frequency greater than about 50 Mhz; a first electrodepositioned on a first side of the quartz crystal; a second electrode,having approximately the same size and shape as the first electrode,positioned on a second side of the quartz crystal; a first polymersensing film in contact with the oscillating region of the quartzcrystal on the first side of the quartz crystal and further in contactwith the first electrode; and a second polymer sensing film in contactwith the oscillating region of the quartz crystal on the second side ofthe quartz crystal and further in contact with the second electrode. 2.The TSM sensor according to claim 1 wherein the operating frequency ofthe polymer coated resonator is about 96 MHz.
 3. The TSM sensoraccording to claim 1, wherein the polymer is selected from the groupconsisting of viscoelectric polymers, polyvinyl acetate, polyvinylpyrrolidone, polyisobutylene, polystyrene, and polystyrenebutadiene. 4.The TSM sensor according to claim 1, wherein the thickness of the firstpolymer sensing film is about equal to the thickness of the secondpolymer sensing film.
 5. The TSM sensor according to claim 1, whereinthe ratio of the diameter of the outer region of the quartz crystal tothe diameter of the oscillating region is about 0.1.
 6. The TSM sensoraccording to claim 1, wherein the thickness of the outer region of thequartz crystal is about 50 micrometers and the thickness of theoscillating region is about 17 micrometers.
 7. The TSM sensor accordingto claim 1, wherein the ratio of the thickness of the polymer sensingfilm to the thickness of the oscillating region is about 0.01.
 8. Amethod of gas or vapor sensing in an analyte comprising the steps of:providing a quartz crystal having an oscillating region of reducedthickness surrounded by an outer region, wherein the outer region iscomparatively thicker than the oscillating region and the oscillatingregion has an operating frequency greater than about 50 Mhz; positioninga first electrode on a first side of the quartz crystal; positioning asecond electrode, having approximately the same size and shape as thefirst electrode, on a second side of the quartz crystal; positioning afirst polymer sensing film in contact with the oscillating region of thequartz crystal on the first side of the quartz crystal and further incontact with the first electrode; positioning a second polymer sensingfilm in contact with the oscillating region of the quartz crystal on thesecond side of the quartz crystal and further in contact with the secondelectrode to form a TSM organic gas or vapor sensor; contacting the TSMsensor with the analyte; and measuring the frequency shift of the sensorupon contact with the analyte wherein a frequency shift is indicative ofthe presence or concentration in the analyte of the gas or vapor to besensed.
 9. The method according to claim 8 wherein the gas or vapor isan organic gas or vapor.
 10. The method according to claim 8 wherein thegas or vapor is selected from the group consisting of benzene, hexane,cyclohexane, heptane, dichloroethane, chloroform, and toluene.
 11. Themethod according to claim 8 wherein the gas or vapor is benzene.
 12. Themethod according to claim 8 wherein the operating frequency of thepolymer coated resonator is about 96 MHz.
 13. The method according toclaim 8, wherein the polymer of the TSM organic gas or vapor sensor isselected from the group consisting of viscoelectric polymers, polyvinylacetate, polyvinyl pyrrolidone, polyisobutylene, polystyrene, andpolystyrenebutadiene.
 14. The method according to claim 8, wherein thethickness of the first polymer sensing film of the TSM organic gas orvapor sensor is about equal to the thickness of the second polymersensing film of the TSM organic gas or vapor sensor.
 15. The methodaccording to claim 8, wherein the ratio of the diameter of the outerregion of the quartz crystal to the diameter of the oscillating regionis about 0.1.
 16. The method according to claim 8, wherein the thicknessof the outer region of the quartz crystal is about 50 micrometers andthe thickness of the oscillating region is about 17 micrometers.
 17. Themethod according to claim 8, wherein the ratio of the thickness of thepolymer sensing film to the thickness of the oscillating region is about0.01.